In certain situations it is best to avoid doing file I/O. For example, a program that runs in an event loop may wish to avoid any potentially blocking operations; reading from a file usually is fast, but can sometimes take arbitrary long. Another example (my specific use case) is a logging library - you don't want to block for an arbitrary amount of time when creating log messages in an application thread (a separate thread would do the writing).

Unfortunately, the traceback.py module reads from files to load the source lines for the traceback (using linecache.py). This means if you want to format a traceback without file I/O you have to either recreate some logic from the standard library, monkey-patch globally, or do a terrible hack you don't want to know about.

It would be better if the there was some way to ask the traceback.py module's functions to not do file I/O. The actual change would be fairly minor I suspect since the formatting functions already support getting None back from linecache.

> Unfortunately, the traceback.py module reads from files to load the source lines for the traceback (using linecache.py).

linecache is supposed to cache the result. When all files generating tracebacks of your project are cached, only os.stat() is called on each file to check if the file has been modified.

But I'm not sure I understood your request: do you want the line content in your traceback or not? If you don't want the line content, the traceback module should be modified to add an option "don't read file content".

Since this issue was created, the traceback module changed considerably, and now we have the TracebackException/StackSummary classes which capture information about an exception for later rendering. The lookup_lines arg controls the timing of the file access: if it is false, this is delayed until render-time.

I believe this solves the use case of the logger - the file access can be delayed till the logger thread renders the exception. I'm not sure I understand the event loop use case, but presumably there, too, the exceptions are not being rendered (because we don't want IO during that loop).